Enhanced sensitivity of graphene gas sensors using molecular doping

Abstract

The sensitivity of a graphene gas sensor to a gas analyte molecule may be significantly enhanced using molecular doping, which may be as effective as substitutional doping and more effective than electric-field doping. In particular, the room temperature sensitivity of NO 2 -doped graphene to NH 3 was measured to be comparable to the sensitivity of graphene doped with substitutional boron atoms and superior to that of undoped graphene by an order of magnitude. The detection limit for NO 2 -doped graphene gas sensors was estimated to be about 200 ppb, which may be improved with extended exposure to NO 2 , compared to a detection limit of about 1.4 ppm for undoped graphene. While the stability analysis of NO 2 -doped graphene sensors indicates that the doping method may not be completely stable, molecular doping is nevertheless a candidate technique for sensitivity improvement by enhancing the initial carrier concentration.

What is claimed is: 
     
       1. A graphene gas sensor, comprising:
 a graphene element molecularly doped with nitrogen dioxide (NO 2 ); 
 a pair of voltage electrodes spaced apart on the graphene element; and 
 a pair of current electrodes at each end of the graphene element, 
 wherein the graphene element is configured to detect ammonia (NH 3 ) based on a conductivity of the graphene element measured using the pair of voltage electrodes and the pair of current electrodes, and wherein a change in the conductivity upon exposure to NH 3  is greater for the graphene element doped with NO 2  than an undoped graphene element. 
 
     
     
       2. The graphene gas sensor of  claim 1 , wherein the graphene gas sensor is included in a complementary metal oxide semiconductor (CMOS) device. 
     
     
       3. The graphene gas sensor of  claim 1 , wherein the graphene element is molecularly doped with NO 2  using 100 ppm of NO 2  in N 2  at 500 Torr pressure and at room temperature. 
     
     
       4. The graphene gas sensor of  claim 3 , wherein the graphene element is molecularly doped for a duration between 10 minutes and 60 minutes. 
     
     
       5. The graphene gas sensor of  claim 1 , wherein the conductivity is measured by:
 applying a current using the pair of current electrodes; 
 measuring a voltage using the pair of voltage electrodes; and 
 determining the conductivity based on the current and the voltage. 
 
     
     
       6. The graphene gas sensor of  claim 5 , wherein the voltage is measured using a field effect transistor. 
     
     
       7. The graphene gas sensor of  claim 1 , wherein the graphene element comprises a single atomic layer of carbon. 
     
     
       8. The graphene gas sensor of  claim 1 , further comprising: a silicon substrate on which the graphene element is situated. 
     
     
       9. A method of detecting ammonia (NH 3 ) gas, the method comprising:
 applying a current to a graphene element molecularly doped with nitrogen dioxide (NO 2 ) using a pair of current electrodes; 
 measuring a voltage across the graphene element using a pair of voltage electrodes; and 
 exposing the graphene element to NH 3  gas while measuring a change in the voltage, wherein the change in the voltage is indicative of the concentration of the NH 3  gas, and wherein the change in voltage is greater for the graphene element doped with NO 2  than an undoped graphene element. 
 
     
     
       10. The method of  claim 9 , wherein the graphene element is implemented in a complementary metal oxide semiconductor (CMOS) device. 
     
     
       11. The method of  claim 9 , further comprising: molecularly doping the graphene element with NO 2  using 100 ppm of NO 2  in N 2  at 500 Torr pressure and at room temperature. 
     
     
       12. The method of  claim 11 , wherein molecularly doping the graphene element further comprises: molecularly doping the graphene element for a duration between 10 minutes and 60 minutes. 
     
     
       13. The method of  claim 9 , further comprising determining a conductivity of the graphene element including:
 applying the current using the pair of current electrodes, spaced apart on the graphene element; 
 measuring a voltage using the pair of voltage electrodes, spaced apart on the graphene element; and 
 determining the conductivity based on the current and the voltage. 
 
     
     
       14. The method of  claim 13 , wherein measuring the voltage further comprises: measuring the voltage using a field effect transistor. 
     
     
       15. The method of  claim 9 , wherein the graphene element comprises a single atomic layer of carbon. 
     
     
       16. The method of  claim 9 , wherein the graphene element is situated on a silicon substrate.